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'[OT] MC1723/LM723 Odd Problem'
1998\10\25@005311 by Sean Breheny

face picon face
Hi all,

I am working on a pic project which will contain a variable voltage supply
using an MC1723CP (motorola's 723 volt reg IC). I have Motorola's datasheet
and I have the circuit hooked up similarly to their datasheet, with one
exception(instead of just feeding the voltage reference into the
non-inverting input of the error amp thru a resistor, I am using a voltage
divider to provide a variable output from 2v to 16v). I am using a 2N3771
TO-3 NPN power transistor as an external series pass transistor to boost
the output current handling capability to about 3 amps. The input to the
regulator is comming from a transformer which is rated at 4 amps and
delivers 24vrms with no load (I get about 34v with the 7000uF cap and
bridge rectifier, no load)

The odd behavior I am getting is that if the transistor gets very hot, pin
5 of the 723 (the non-inverting input of the op-amp!) starts sourcing lots
of current into my voltage divider network(measured at about 150uA(normal
is about 1uA), enough to shift the nominal 10k impedance network from 5.1v
to 6.9v - 8.8 v, even higher than the 7.0v reference which is supposed to
be the most positive point attached to the divider).This jump in voltage
causes the output to shoot to the full 34v of Vin. The transistor is pretty
far away from the IC and the IC is not getting even warm. None of the other
parts are getting warm either. Yet, it does seem to be heat related because
it only happens when the transistor is very hot. The net effect is that the
voltage shoots up on the output when I disconnect the load, and it only
returns down to the normal regulated value after the transistor cools a bit
(it returns down somewhat faster if I blow across the heatsink).

I realize that my transistor is getting too hot and I will need a bigger
heatsink, but I am very curious to know why this is causing the 723 to
react this way, when it is reasonably thermally isolated from it. I could
understand also if the pins connected to the transistor were doing
something strange (due to different current demands, etc.) But this pin is
not connected to the output circuit at all!

This circuit is also on a breadboard which I understand is HIGHLY
suboptimal for a high current device and I will be soldering it soon, but I
want to rough out the major bugs first.

Also, nothing seems to be oscillating or anything like that because nothing
changes when I touch the wires or change their positions relative to each
other.

I have tried several 723's with different date codes, and all showed the
same behavior. Below is ascii art of my schematic:



                +--------------------------------+
                |    +------------------+        |
                |    |        723       |       /
Vin(unreg)-------+----|12(Vcc)   (Vout)10|------[   NPN(2N3771) with small
TO-220 heat sink
                |----|11(Col. of        |       >
                     |   internal       |        \
                     |   pass xistor)   |         +----------Vout(reg)
                +----|6(Vref=7v)        |         |
                |    |     (Curr lim)  2|---------+
            20KPOT-+-|5(V+ of amp)      |         |
                |  | |     (Curr sense)3|---------+
                |  | |                  |        13K
               1K5 | |                  |         |
                |  | |      (V- of amp)4|---------+----470pF--+
                | C1 |  7(Vee)(Fcomp)13 |         |           |
                |  | +------------------+        10K          |
               GND GND  |             |           |           |
                       GND            |          GND          |
                                      +-----------------------+

NOTE: C1=0.1uF(omitting C1 makes no difference with the problem)
Current limiting is disabled because right now I don't have a .17 ohm power
resistor or
enough 1 ohmers to make one.
The "T" of the word POT is the wiper.
I also tried 100pF for the 470pF cap and it made no difference in the
problem but did make the
output spike a bit more under normal (non-transistor overheat) conditions.

Other than the problem, the circuit is behaving quite well (about 2%
regulation, which is not bad for the fact that it is on an old breadboard
and there are several alligator clips in the circuit!) The only other
slightly strange problem is that at very low output voltage settings
(around 2v), I get a positive spike when a high current load is attached.
This lessened quite a bit when I changed the 100pF cap to 470pF. 100pF is
Motorola's recommended value, and 470pF is what Paul Webster recommends on
his FAQ on the 723. I have also tried using the resistor in series with the
Current Sense lead as suggested by Paul and it makes no difference in my
case. What I did find out, however, is that the problem is much less in
magnitude if I completely disconnect the Current sense and current limit pins.

Any ideas?? I would greatly appreciate it if someone could give me some
suggestions.
Sorry that this is so long, it just seems that there is no sense in sending
out a question if the problem isn't described well enough.

Thanks very much,

Sean

+-------------------------------+
| Sean Breheny                  |
| Amateur Radio Callsign: KA3YXM|
| Electrical Engineering Student|
+-------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
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1998\10\25@062635 by Valter Gruntar

flavicon
face
I think that the problem is just in heating the power transistor. When it's so h
ot
as you say, it doesn't mater what is the Voltage on the BASE of transistor. It
conducting anyway cause of heating. You just have to put it on bigger cooler or
reduce the unregulated voltage from transformer. The power heating on the
transistor you can calculate:

(Vin(unregulated) - Vout(regulated) ) * I(current in amperes) = W(power in watts
)

hope this help you

Valter

Sean Breheny wrote:

{Quote hidden}

1998\10\25@095226 by Mike Keitz

picon face
On Sun, 25 Oct 1998 00:51:13 -0400 Sean Breheny <shb7spamKILLspamCORNELL.EDU>
writes:

>Any ideas?? I would greatly appreciate it if someone could give me
>some
>suggestions.
>Sorry that this is so long, it just seems that there is no sense in
>sending
>out a question if the problem isn't described well enough.

The problem is well described.  Here's what I think is going on.  The
voltage at pin 5 rising is a symptom of a larger problem -- the output
voltage is rising to 34V out of control.  When this happens, the voltage
at pin 6 rises enough to exceed the 5V input differential voltage
specification of 5V.  The differential amplifer transistor in the 723
breaks down and starts sourcing current out of pin 5.  Again, this is
only a symptom, not the problem (though it could damage the chip, so it
needs to be prevented).


So why is the output voltage rising out of control?  Because the
transistor is getting hot (You already knew that too).  When a transistor
gets hot, it starts to leak from collector to base and turn on by itself.
The 723's output stage is unable to sink current away from the base,
only source it, so it can't control this leakage.  You need to provide a
path for the leakage current.  Usually a resistor of 100 ohms or so is
connected from the base to emitter of power transistors used in circuits
with single-direction drive like this.  Add the resistor and see if that
helps.  Of course you should keep the transistor cool in the first place,
but that's not an excuse for not having the resistor.



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1998\10\25@114007 by Thomas McGahee

flavicon
face
Sean,
The base root of your problem is the heating of the T03 power transistor.
When it overheats it will go into a condition called thermal
runaway. The heat causes the transistor to turn ON due to free electrons
caused by the heat. Once the transistor gets too hot the extra free
electrons are enough to keep the transistor turned on, even if you remove
all base drive. One method of pulling the transistor OFF is to add a
resistor between the base and emitter of the power transistor. A typical
value is 100 ohms.

But adding the resistor only puts a big band-aid on the problem. As long
as the over-heating remains you will experience some level of problem,
because the extra voltage that appears at the base of the power
transistor is getting fed back through the 723 regulator and causes pin
5's voltage to rise. You could add a diode at the base of the power
transistor to alleviate this *symptom", but the base problem would still
remain.

You can put one of those small fans on the heat sink of the power
transistor. That will greatly reduce the tempertature, and may even
solve you problem.... but the *real* problem would still remain, because
you have not eliminated the SOURCE of the heating problem.

Heat is a function of power dissipated. In your case you have 34 volts
(no-load), and probably something like 22 volts at 3 amp load. Let's
say you have the circuit supplying 5 volts at 2 amps. That means the
poor power transistor sees somewhere between 17 and 29 volts across itself
and 2 amps through itself. That is 34 to 58 watts of heat it has to
dissipate. You might have a power transistor rated at 100 watts, but
read the spec sheets carefully and you will see that that is at a certain
case temperature.

You want to reduce, if possible, the amount of voltage appearing ACROSS
the transistor. Replace the power transformer with something closer to
the maximum voltage you desire. That hits the problem at the actual SOURCE
of the problem. By the way, note that it is when you dial the LOWEST
output voltages that the transistor has to take the HIGHEST voltages.

Other things that will improve any such circuit include :
Use as large or massive a heat sink as possible
Use heat sinks with large surface areas (fins!)
Use forced-air cooling, and mount the fan as close as possible
Use multiple power transistors in parallel with emitter resistors
 (this will allow load-sharing)

Hope this helps.
Fr. Tom McGahee

----------
{Quote hidden}

pins.
{Quote hidden}

1998\10\25@143159 by Sean Breheny

face picon face
Hi all,

Thanks to everyone who responded. You were right on the money, that power
transistor is turning itself fully on due to the excess heat. I determined
this by causing the condition and then removing all base drive and the
output stayed at full 34V. Adding a 100 ohm resistor from base to emitter
helped, but did not solve the problem entirely (it still shoots up to 34V
but it only takes about 10 sec to come back down instead of 30 sec without
a resistor). BTW, I never knew exactly HOW VERY HOT that transistor is
getting! I have no lab thermometer with me, but just as a test, I put a
large drop of water on the heat sink and then put a 1.5 amp load on the
circuit. Within 20 sec, the drop began to boil and went from about .5 ml of
liquid to zero in about 4 secs!!!!! As an aside, the heat in my dorm room
is not working right now and I have just submitted a service request(it is
starting to get cold enough up here in upstate NY to need this!). I think I
may keep this version of my circuit, it makes a very nice little hand
warmer <VBG>

So, I guess the point is that I need a MUCH bigger heat sink, and possibly
a small fan. To answer some questions asked of me, it is a silicon
transistor(2N3771). It is rated 150W dissipation at 25deg C, but at about
150deg C (where I'm guessing mine is at right now!), it can only dissipate
20W safely. In my test, I was dissipating about 15-20W, so I was close to
harming it.

I never expected the heat sink on it now to be good enough in the long run
(it is only a little TO-220 heat sink with some heat sink compound on its
flat side, laying on top of the TO-3 can), but I though I could do some
quick tests with it. Now I realize that these types of circuits are even
more thermally dependent than I thought. A good lesson!

As for changing the unregulated input: my end goal is to have a 2V to 16V
3amp supply, so I figured that I needed an input voltage that had somewhat
of a margin, even above the 16V max. I have drawn at most 2 amps from it
now, and I get 22V out of the unregulated portion, so I'm guessing that it
may drop as low as 19 or so at 3 amps, so I think that I DO need this much
margin to get good regulation at 16V.

The suggestion of using a switching regulator first, then followed by the
linear one seems like a great suggestion, except that I want to keep the
cost down and fit this in a relatively small space (the transformers take
up alot!). No, this is not going to be a product to be marketed, but I
already have a case drilled for it and not much money left over!

I wanted to avoid having to parallel several pass transistors because space
is at a bit of a premium in this device. I'm hoping that with a decent heat
sink, the problem will stop showing up.

Thanks again, specifically:

Paul Webster
Valter Gruntar
Mike Keitz
Fr. McGahee
Peter Peres



+-------------------------------+
| Sean Breheny                  |
| Amateur Radio Callsign: KA3YXM|
| Electrical Engineering Student|
+-------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
@spam@shb7KILLspamspamcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1998\10\25@170622 by Mike Keitz

picon face
On Sun, 25 Oct 1998 14:31:38 -0500 Sean Breheny <KILLspamshb7KILLspamspamCORNELL.EDU>
writes:

>As for changing the unregulated input: my end goal is to have a 2V to
>16V
>3amp supply, so I figured that I needed an input voltage that had
>somewhat
>of a margin, even above the 16V max. I have drawn at most 2 amps from
>it
>now, and I get 22V out of the unregulated portion, so I'm guessing
>that it
>may drop as low as 19 or so at 3 amps, so I think that I DO need this
>much
>margin to get good regulation at 16V.

A common approach to this situation is to change taps on the transformer
according to the approximate output voltage.  A comparator on the output
connected to a relay would work.  Using two possible input voltages
instead of just one will help a lot.  The comparator circuit should have
hysteresis so the relay doesn't chatter on and off at the threshold
output voltage.  You want to control based on the actual output voltage
not the desired output voltage in order to keep the voltage across the
transistor low in case the supply is in current limit.

To dissipate 20W, you'll need a regular finned heat sink attached to the
outside of the cabinet, or if it's inside provide lots of vent holes in
the cabinet above and below it.  It would also be a good idea to fit a
thernal sensor or switch to shut the supply down in case of overheating.



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1998\10\25@173110 by Sean Breheny

face picon face
Hi Mike,

At 05:03 PM 10/25/98 -0500, you wrote:
>A common approach to this situation is to change taps on the transformer
>according to the approximate output voltage.  A comparator on the output
>connected to a relay would work.  Using two possible input voltages
>instead of just one will help a lot.  The comparator circuit should have
>hysteresis so the relay doesn't chatter on and off at the threshold
>output voltage.  You want to control based on the actual output voltage
>not the desired output voltage in order to keep the voltage across the
>transistor low in case the supply is in current limit.

That's a great idea! I think I do have the stuff on hand to do that! It
would be kinda neat, too, to have the supply make a little "click" as you
crossed over, say, the 7v point. (My transformer is center tapped, so I'll
have to check and see, but I'd immagine that I should switch at about 7v
output)

>
>To dissipate 20W, you'll need a regular finned heat sink attached to the
>outside of the cabinet, or if it's inside provide lots of vent holes in
>the cabinet above and below it.  It would also be a good idea to fit a
>thernal sensor or switch to shut the supply down in case of overheating.

Well, originally, I had hoped to keep everything inside, but I doub't that
there's going to be enough space inside for two of such heat sinks (the
supply is actually TWO power supplies, one inverted, to give +/-
independently adjustable supply) Could you guide me as to the approximate
size of heat sink needed for about 20W? I know I could go by thermal
resistance to ambient, but I don't see any sinks in Mouser's catalog with a
therm resistance of less than 5 deg C/Watt, so I'd have 100 deg C (at
least,not including therm resistance of junction to case, and thermal
grease). When I look at surplus dealers, they don't list therm resistance,
but do give size. I think some of these are big enough, but I'm not sure.

I guess the best strategy for thermal shut-down would be a latching relay
that latches to the normally-open position  when the temp goes too high and
breaks connection between the unregulated portion and the regulator? That
way, the device would have to be manually cycled before it would reset.
What temperature would be best to sense, the heat sink's temp?

Thanks very much,

Sean



>
>
>
>___________________________________________________________________
>You don't need to buy Internet access to use free Internet e-mail.
>Get completely free e-mail from Juno at http://www.juno.com/getjuno.html
>or call Juno at (800) 654-JUNO [654-5866]
>
+-------------------------------+
| Sean Breheny                  |
| Amateur Radio Callsign: KA3YXM|
| Electrical Engineering Student|
+-------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
RemoveMEshb7TakeThisOuTspamcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1998\10\25@175556 by Sten Dahlgren

flavicon
face
Sean Breheny wrote:
>
> Hi Mike,
>
> At 05:03 PM 10/25/98 -0500, you wrote:
> >A common approach to this situation is to change taps on the transformer
> >according to the approximate output voltage.  A comparator on the output

--snip--

> Thanks very much,
>
> Sean
>
Another way may be to use a 486 cpu heatsink with fan. Cheap and very
small.

/Sten

1998\10\25@180426 by Mike Keitz

picon face
On Sun, 25 Oct 1998 14:31:38 -0500 Sean Breheny <spamBeGoneshb7spamBeGonespamCORNELL.EDU>
writes:
I needed an input voltage that had
>somewhat
>of a margin, even above the 16V max.

Another standard design is to use another supply to operate the 723 at
8-10V above the main unregulated input that supplies the collector of the
pass transistor.  Either another transformer winding or a capacitive
voltage doubler (working at 60Hz) can be used, only 200 mA or so is
needed.  This allows the 723 to drive the transistor fully into
saturation so the regulated output can closely approach the unregulated
input.  Now it's important to consider that the 723's output may burn out
if not enough voltage is available at the pass transistor to reach the
regulated output.  So additional protection may be needed.  Using a FET
instead of a transistor for the pass element would solve many of these
problems but riase the cost some.  A boosted 723 input voltage would be
needed for sure.

Another circuit uses a PNP pass transistor.  This has the same problem of
needing to limit the drive current somehow.  Additionally the circuit is
less stable, and the drive current flows to ground rather than to the
regulated output.

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1998\10\25@191339 by Dwayne Reid

flavicon
face
>I am working on a pic project which will contain a variable voltage supply
>using an MC1723CP (motorola's 723 volt reg IC).

   <snip>

>The odd behavior I am getting is that if the transistor gets very hot, pin
>5 of the 723 (the non-inverting input of the op-amp!) starts sourcing lots
>of current into my voltage divider network(measured at about 150uA(normal
>is about 1uA), enough to shift the nominal 10k impedance network from 5.1v
>to 6.9v - 8.8 v, even higher than the 7.0v reference which is supposed to
>be the most positive point attached to the divider).This jump in voltage
>causes the output to shoot to the full 34v of Vin.

Um . . . I think that you have it backwards!  What **I** think is happening
is that the output transistor is starting to 'run away' on you and conduct
more than it is supposed to.  It conducts more, so your output voltage is
higher than it is supposed to be, so the voltage at the NI input of the
op-amp is higher than it is supposed to be.

A couple of ways to check this: get the transistor nice and hot, then snip
the wire going to its base.  If the output voltage is still too high, you
have a dud transistor.

A couple of things you can try - a 47R or 100R resistor from B-E might help.
It is possible the transistor is oscillating - a 47R in series with the base
(right at the transistor) will stop that but may limit your maximum output
current at higher output voltages (you lose headroom because of the voltage
drop across the resistor when the transistor needs lots of base current).

I'd bet that the resistor from B-E does the trick.  Give it a try.

dwayne



Dwayne Reid   <TakeThisOuTdwaynerEraseMEspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1998\10\26@125557 by Peter L. Peres

picon face
On Sun, 25 Oct 1998, Sten Dahlgren wrote:

> Sean Breheny wrote:
> >
> > Hi Mike,
> >
> > At 05:03 PM 10/25/98 -0500, you wrote:
> > >A common approach to this situation is to change taps on the transformer
> > >according to the approximate output voltage.  A comparator on the output
>
> --snip--
>
> > Thanks very much,
> >
> > Sean
> >
> Another way may be to use a 486 cpu heatsink with fan. Cheap and very
> small.

And, unfortunately, not very efficient. A P2 heatsink is more like it. The
P2 fries about 19 Watts when running Windows if I'm no wrong. (It fries
much less under Linux, BSD and NT because of proper HALT instruction use).

Peter

1998\10\26@173020 by Junk kontot

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